1. Technical Field
Methods for forming dual gate oxide layers, and more particularly, methods for fabricating semiconductor device having dual gate oxide layers are disclosed.
2. Description of the Related Art Generally, a silicon dioxide SiO2 layer, which is grown by a thermal process, is used for a gate oxide layer. In general tendency of the semiconductor device design, the thickness of the gate oxide layer is below the range of 25 Å to 30 Å, which is the maximum tunneling limit of the SiO2 layer.
However, a threshold voltage Vt in the cell is higher than the threshold voltage Vt in the peripheral circuit, because of the refresh. Therefore, a high gate voltage needs to be applied to the cell transistor so that the electrical characteristic is not deteriorated.
To enhance the characteristic of the cell transistor, a dual gate oxide formation method needs to be applied to the cell transistor in order to increase the thickness of the gate oxide layer.
Among the several techniques for forming the dual gate oxide layer, two methods are widely used. One of them is a method for forming the dual gate oxide layer by eliminating the gate oxide layer in some part, then oxidizing and forming the dual gate oxide layer again. Another method includes forming the dual gate oxide layer by ion implanting nitrogen in some part in order to idle the growth of the gate oxide layer.
However, the first method has a problem that a semiconductor substrate is damaged because the high heat process is applied twice to the semiconductor in order to form the dual gate oxide layer. The second method has a similar problem that the semiconductor substrate is damaged by ion implanting nitrogen.
Especially if the semiconductor substrate is damaged, deterioration of the channel mobility results.